In the case of lignocellulosic biomass, a very promising integrated configuration for bioethanol production is the inclusion of pentose fermentation in the SSF, as illustrated in Figure 9.6. This process is known as simultaneous saccharifica­tion and co-fermentation (SSCF). This configuration implies a higher degree of intensification through its reaction-reaction integration. In this case, the hydro­lysis of cellulose, the fermentation of glucose released, and the fermentation of pentoses present in the feed stream is simultaneously accomplished as a single unit. Besides the effectiveness of employed cellulases, the key factor in SSCF is the utilization of an efficient ethanol-producing microorganism with the ability of assimilating not only hexoses (mainly glucose), but also pentoses (mainly xylose) released during the pretreatment step as a result of the hemicellulose hydrolysis. In nature, there exist several microorganisms able to assimilate both types of sug­ars, but their ethanol yields are low as is their growth rate. Therefore, genetically modified microorganisms have been developed and successfully proven in SSCF processes for ethanol production from lignocellulosic materials.

Lignocellulosic

— Ethanol

In an initial stage, the co-fermentation of mixed cultures was studied (Cardona and Sanchez, 2007). For example, the co-culture of Pichia stipitis and Brettanomyces clausennii has been employed for the SSCF of aspen at 38°C and pH of 4.8 yielding 369 L EtOH per ton of aspen during 48 h batch process, as reported by Olsson and Hahn-Hagerdal (1996). In this configuration, it is neces­sary that both fermenting microorganisms have compatible in terms of operating pH and temperature. Chandrakant and Bisaria (1998) suggest that a combination of C. shehatae and S. cerevisiae is suitable for this kind of process.

The actual SSCF process has been demonstrated in the case of ethanol produc­tion from yellow poplar through a bench-scale integrated process that included the dilute-acid pretreatment of feedstock, conditioning of hydrolyzate for fermen­tation, and a batch SSCF (McMillan et al., 1999). In this case, the recombinant bacterium Zymomonas mobilis assimilating xylose was used. SSCF is the process on which is based the technology designed as a model process by the National Renewable Energy Laboratory (NREL) for production of fuel ethanol from aspen wood chips (Wooley et al., 1999b) and corn stover (Aden et al., 2002). In this design, the utilization of recombinant Z. mobilis exhibiting a glucose conversion to ethanol of 92% and a xylose conversion to ethanol of 85% is proposed. It is projected that SSCF can be carried out in a continuous regime with a residence time for the entire system of cascade fermenters of 7 d at 30°C (Cardona and Sanchez, 2007).

As in the case of SSF of biomass, the development of microbial strains able to grow at elevated temperatures can improve the technoeconomic indicators of the process (Cardona and Sanchez, 2007). Thus, ethanol-producing microorganisms capable of assimilating both types of sugars at temperatures higher than 50°C could reduce the cellulase costs by half, taking into account that a 20°C increase during saccharification can lead to double cellulose hydrolysis rate (Wooley et al., 1999a). Three examples of SSCF of lignocellulosic materials pretreated are presented in Table 9.4.